A method of lowering the power absorbed by an interface circuit, in the "power-down" state thereof, as incorporated to a telephone exchange and connected to a telephone subscriber line, being of a type which comprises a monitoring circuit portion connected between the line and the exchange. The method involves the steps of,detecting the polarization level of a conductor in the line,comparing that level with a reference value by means of a comparator having an input connected to the line and an output connected to the input of the monitoring circuit portion,switching the interface circuit to a standby state on a higher level than the reference value being sensed,once again detecting the polarization level of the line, this time through the interface circuit, andactivating the telephone exchange when, on completion of the second detection step, the polarization level stays above the reference value.